1. Field of the Invention
This invention relates generally to implantable medical device systems, and, more particularly, to implantable medical device systems including implantable medical devices for providing cardiac arrhythmia therapies to patients having the devices implanted within.
2. Description of the Related Art
Myocardial or cardiac ischemia is an intermediate condition in coronary artery disease during which the heart tissue is slowly or suddenly starved of oxygen and other nutrients. Myocardial ischemia is usually caused by blockage of a coronary artery, usually due to atherosclerotic plaque. Myocardial ischemia may also be caused by blood clots (which tend to form on plaque), artery spasms or contractions, or any of the above conditions in combination. If adequate blood flow to affected heart tissue is not restored, the affected heart tissue will die. When blood flow is completely blocked to the heart, myocardial ischemia can lead to myocardial infarction (i.e., a heart attack).
Myocardial ischemia can be symptomatic or silent. Symptomatic myocardial ischemia is characterized by chest pain (i.e., “angina pectoris” or simply “angina”), especially during physical exertion. People with angina are at risk of having a heart attack. Those suffering silent myocardial ischemia have no signs, and are typically at greater risk of having a heart attack with no warning than those with symptomatic myocardial ischemia.
Non-invasive tests used to diagnose myocardial ischemia and other types of heart disease include resting electrocardiogram (ECG), ambulatory ECG (i.e., Holter monitoring), exercise stress test (i.e., exercise ECG), and echocardiography. Invasive diagnostic tests requiring intravenous injection include pharmacological stress tests and nuclear imaging techniques. Other invasive imaging techniques include transesophageal echocardiography and coronary angiography (i.e., cardiac catheterization).
In an exercise stress test, a patient walks on a treadmill, or pedals an exercise bicycle, at increasingly higher levels of physical exertion. The patient's heart rate and blood pressure increase with the levels of physical exertion. In a pharmacological stress test, a drug (e.g., dipyridamole, dobutamine, adenosine, etc.) is administered intravenously to increase heart rate and blood pressure in a manner similar to the effects of physical exertion. During exercise and pharmacological stress tests, the patient's heart rate and blood pressure are monitored. If the patient's heart tissue does not receive needed amounts of oxygen and nutrients, the patient experiences myocardial ischemia.
Non-invasive and free of chemical side effects, exercise stress tests are typically preferred over pharmacological stress tests. Invasive pharmacological stress tests are usually performed on patients that cannot tolerate exercise stress tests (e.g., patients with physical limitations such as back trouble, joint disease, marked fatigue, etc.).
Exercise stress tests are, however, often difficult and costly to perform. For example, an exercise stress test requires special equipment, several technicians, and patient training. Stress tests typically have a target heart rate which is usually determined using the formula 0.85·(220-age). To undergo an exercise stress test, a patient must be both willing and able to physically exert themselves to the target heart rate. Many patients have a difficult time reaching the target heart rate, especially in pharmacological stress tests.
Transesophageal atrial pacing has also been used to elevate heart rates and test for myocardial ischemia. Transesophageal atrial pacing takes advantage of the anatomical proximity of the esophagus to the left atrium of the heart for minimally invasive heart stimulation. In preparation for transesophageal atrial pacing, a patient swallows an electrode connected to one end of a lead. While in the esophagus, the electrode is positioned near the left atrium of the patient's heart by adjusting the lead length. A stimulator is coupled to the other end of the lead, and produces electrical pulses which electrically stimulate (i.e., “pace”) the left atrium of the heart. In addition to providing a method for temporary atrial pacing in patients whose heart rates are too slow or irregular to meet the demands of their bodies (i.e., patients with bradycardia), transesophageal pacing also offers an alternative method for elevating heart rates for diagnosis of myocardial ischemia and other types of heart disease.
The present invention is directed to a system and method facilitating noninvasive testing for myocardial ischemia in patients with an implantable medical device (e.g., an implantable pulse generator or IPG, an implantable cardioverter defibrillator or ICD, etc.). The system and method may eliminate the need for invasive tests for myocardial ischemia in such patients.